1. Field of the Invention
The present invention relates to a valve timing control system for varying valve timing of intake and exhaust valves of an internal combustion engine for a motor vehicle according to a vehicle driving condition.
2. Description of the Related Art
Various valve timing control systems have been proposed and in practical use for the purpose of further improving performance of internal combustion engines. Japanese Patent Unexamined Publication No. 5(1993)-1514 discloses a typical valve timing control system which employs a pair of spring clutches and a selector for switching operation of the spring clutches. This conventional valve timing system is arranged to vary the valve timing by controlling a phase varying device installed between a sprocket and a camshaft. The phase varying device for preferably varying the valve timing into an advanced state or retarded state as shown in FIG. 30 has a pair of spring clutches coaxial with the camshaft and a clutch switching mechanism for controlling holding states of the spring clutches.
Japanese Patent Unexamined Publication No. 9(1997)-250309 discloses another conventional valve timing control system which comprises a rotational phase controlling device constituted by a solenoid clutch and a sun-and-planet gear set. This conventional valve timing control system keeps a stationary holding state between a sprocket and a camshaft by applying a biasing force of a plate spring to the solenoid clutch.